The swimming behavior of laval manila clam Ruditapes philippinarum and laval hen clam Pseudocardium sachalinense can be modeled by repeating the rise by a spiral movement and the swim stop generated intermittent settling. The planktonic larva of bottom-clinging stage increases the swimming speed and the diameter and the fall speed of a spiral movement with growth, and the rotation cycle of a spiral movement has decreased. If the particle diameter of the sediment is larger than the shell length, the planktonic larva is caught in the pore of the sediment when reaching bottom by the settling. If the particle size of the sediment is significant small compared with the diameter of a spiral movement, the planktonic larva caught in the sediment pore is gradually opened from the pore while repeating the surface of the sediment and the head-on collision. On the other hand, if the particle size of the sediment is almost equal to a spiral diameter, the planktonic larva keeps being caught in the sediment because a spiral movement becomes possible in the sediment pore. Larva's dispersion process after the bottom-clinging can be formulated by the advection dispersion equation, and can be evaluated by dispersion coefficient.
